1. Field of Invention
The invention relates to an infinitely variable transmission (IVT) that establishes torque circulation in combination with a planetary gear set and, more specifically, to an infinitely variable transmission that establishes a neutral state using the torque circulation.
2. Description of Related Art
Recently, because of demands for significant improvements in fuel consumption as well as driving performance, an automatic transmission equipped with a belt-type continuously variable transmission (CVT) is drawing attention as being used for motor vehicles.
Conventionally, Japanese Patent Application Laid-Open No. 6-331000 discloses an infinitely variable transmission (IVT) having a continuously variable transmission, a constant-speed mechanism and a planetary gear mechanism for synthesizing power transmitted from the continuously variable transmission and the constant-speed mechanism. The infinitely variable transmission generates a torque circulation. Thereby the amplification of the speed-change amplitude is achieved. In this infinitely variable transmission, the engine output is transmitted to a carrier through the constant-speed mechanism, and it is also transmitted to a sun gear through the continuously variable transmission and a first (low) clutch or one-way clutch. In this state, the torque circulation occurs in the infinitely variable transmission. As the speed-change ratio of the continuously variable transmission increases from a small value (O/D) to a large value (U/D), the speed-change ratio of the output shaft of the infinitely variable transmission to the ring gear also changes. Namely, it changes from a value corresponding to the rearward movement, through infinity (the output rotation is null) and a large value corresponding to the forward movement, and to a small value corresponding to the forward movement. Furthermore, when the first clutch or one-way clutch is disengaged and a second (high) clutch is engaged, the rotation from the continuously variable transmission is directly transmitted to the output shaft. As the speed-change ratio of the continuously variable transmission decreases from a large value (U/D) to a small value (O/D), the rotational ratio of the output shaft also decreases from a large value (U/D) to a small value (O/D).
The infinitely variable transmission for generating torque circulation makes it possible to locate geometrically a gear neutral position where the rotation of the output shaft becomes zero. This is achieved by setting the speed-change ratio of the continuously variable transmission to a predetermined value determined by the gear ratio of the planetary gear mechanism. Therefore this infinitely variable transmission theoretically obviates starting devices, such as a clutch or a torque converter.
When this infinitely variable transmission with torque circulation is in a drive state where torque is transmitted from the engine to the wheels in the vicinity of the gear neutral position, it is able to obtain a high torque ratio. However, when this infinitely variable transmission is in a coasting state where torque is transmitted from the wheels to the engine, it obtains an extraordinarily large gear ratio (gear ratio at a speed stage lower than the first speed stage of a conventional automatic transmission). This state generates a great engine braking effect and hinders smooth torque transmission based on the transmission efficiency of the transmission.
In particular, when the vehicle needs to start, move slowly forward and stop repeatedly in traffic congestion, it is necessary to turn the accelerator pedal on and off repeatedly with the gear ratio being within a range not far from the gear neutral position. This leads to a very awkward movement of the vehicle, which is annoying to the driver.
In order to prevent such a problem, it may be considered to provide a one-way clutch in a power transmission path in order to eliminate the power transmission caused by the coasting state or temporarily cause the first (low) clutch to be disengaged or slip. However, this would require the entire transmission to be large or a complicated control mechanism for switching over the operation of the one-way clutch or controlling slipping of the first clutch.